Since the capacity of optical communication systems is required to be increasingly larger, spectrum utilization efficiency is incessantly demanded to be higher. Various complicated modulation schemes already mature in the field of wireless technology will be gradually used by optical communication systems, and the emergence of high-speed digital signal processing chips and coherent optical communication technique makes it possible to transmit optical signals having complicated modulation schemes. In comparison with the conventional intensity modulation (OOK) technique, the new modulation schemes are capable of loading information to the phase and polarization state of light, and the modulated signals can be multilevel signals. The MZ (Mach-Zehnder) modulator is the basic unit for achieving these modulation schemes.
FIG. 1 is a schematic diagram illustrating two types of conventional MZ modulators. As shown by reference numeral 101 in FIG. 1, a single-branch MZ modulator is capable of loading information contained in a driving signal on the phase or intensity of an input optical signal under a certain bias voltage. As further shown by reference numeral 102 in FIG. 1, a multi-branch MZ modulator (MZ vector modulator) is basically structured by consisting of two branches (each referred to as an interference arm) and a 90° phase shifter 103. The two upper and lower interference arms are each provided with one single-branch MZ modulator. The 90° phase shifter 103 ensures that the two branches operate in the orthogonal state. By proper setting of the respective bias voltage, the MZ vector modulator can achieve ideal QPSK modulation. If the polarization multiplexing technique is further employed, the spectrum utilization efficiency of the DP-QPSK signal as generated can be four times as high as that of the conventional OOK modulation scheme.
The current DP-QPSK has been publicly acknowledged as going to become the mainstream modulation scheme for the next-generation optical communication system. On the other hand, in order to compensate for damages inherent in the system such as chromatic aberration, in-channel nonlinearity and passband narrowing effect, it is also possible to use the MZ (vector) modulator to perform electric domain predistortion on the signal. That is to say, the predistorted driving signal is transmitted, a predistorted optical signal is obtained after passing through the MZ (vector) modulator, and a non-distorted or approximately non-distorted signal is obtained at the receiver end after passing through a link with distortion. The MZ modulator can flexibly generate various complicated modulation schemes, and can achieve quasi-linear modulation at the same time, so that it is an indispensable component part in the next-generation optical communication system.
The optical signal output from the MZ modulator as a transfer function of the driving voltage (also referred to as MZ modulator transfer function) is controlled by the bias voltage. According to difference in bias voltages, the MZ modulator can operate on a peak point, a light extinction point, and an orthogonal point. FIG. 2 exemplarily illustrates the relationship between the transfer function and the operating point of the MZ modulator. As shown in FIG. 2, the modulator operates on the light extinction point usually when the modulation scheme is QPSK or in the case of pre-compensation. The operating point of the modulator directly affects system performance, especially so with regard to system performance of the pre-compensation system. In actual circumstances, external factors (such as temperature and pressure) directly cause the operating point of the MZ modulator to shift. To ensure stable operation over a long period of time, the bias voltage is usually needed to be controlled for the MZ vector modulator. A method was proposed in US unexamined Patent application publication US 2007/0212075 disclosed on Sep. 13, 2007 for such bias control, to enable the MZ modulator to be biased on the light extinction point, namely to set the power of optical carrier as zero. US unexamined Patent application publication US 2007/0212075 is hereby incorporated by reference, as if it were completely enunciated herein. US unexamined Patent application publication 2007/0212075 directly detects the power of the carrier by the method of adding, at the output end of the modulator, an optical filter with extremely narrow band, thus effectively enhancing the precision of the feedback signal varying with the bias voltage. However, during the process of developing the present invention, inventors of the present invention found the method proposed in the above US unexamined Patent application publication is defective in the facts that the optical filter with extremely narrow band is itself very expensive, and that it is very difficult to ensure alignment of the central wavelength of the filter with respect to the wavelength of the laser.